The total contribution margin a customer generates across their whole relationship with you, not their total revenue. Revenue-based LTV flatters you; margin-based LTV tells the truth. LTV only means something next to CAC, which is why the LTV:CAC ratio is the number investors and acquirers actually look at.
Benchmark. An LTV:CAC ratio of 3:1 is the rule of thumb for a healthy business. Below 1:1 you lose money on every customer; above 5:1 usually means you are underinvesting in growth.